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would enable VIA-FLIC measurements of the height of labeled positions in theion channel.

482-Pos Board B268New Insights into the Membrane Mechanism of Action of Amphotericin Bfrom Molecular Dynamics SimulationsAnna Neumann, Jacek Czub, Maciej Baginski.Gdansk University of Technology, Gdansk, Poland.Amphotericin B (AmB) is a well-known polyene antifungal antibiotic that actsby forming channels in cell membranes. It is known that membrane sterols areessential for the AmB biological activity. The selective toxicity of AmB forfungal cells is attributed to the fact that it is more potent against fungal cellmembranes containing ergosterol than against the mammalian membraneswith cholesterol. There are two non-contradictory models suggesting whatthis may result from: either from stronger AmB-sterol interactions in case ofergosterol membranes or from a more appropriate environment for the forma-tion of a functional pore provided by more ordered ergosterol-containingmembranes.To elucidate the molecular nature of the AmB higher selectivity for ergosterol-containing membranes than for cholesterol-containing ones, we used computa-tional methods and studied (1) the AmB monomer insertion to membranes ofdifferent composition (containing or not 30% of ergosterol or cholesterol),(2) the formation of the putative AmB/sterol complexes in a lipid bilayer and(3) the formation of the AmB dimers in a lipid bilayer. To obtain the equilib-rium description of these processes the free energy profiles were calculated foreach of the studied systems. The significant differences in the affinity of AmBfor different membranes have been found. The results indicate that the differentbehavior patterns of AmB in different membranes is sterol-dependent. Themeaning of these differences for the mechanism of action of AmB and, morespecifically, for the mechanism of channel formation in different membranesis discussed. The data obtained allowed us also to suggest a possible originof the increased selectivity towards ergosterol-containingmembranes of a novelclass of less toxic AmB derivatives.

483-Pos Board B269STED-FCS on Near-Critical Lipid MembranesJens Ehrig, Eugene P. Petrov, Petra Schwille.Technische Universitaet Dresden, Dresden, Germany.Dynamic phase separation in cell membranes is believed to play an importantrole in many membrane-associated cellular processes. This microheterogeneityis one of the reasons for anomalous diffusion of lipid molecules which is fre-quently observed in cell membranes. We have recently shown via Monte Carlosimulations that the presence of near-critical fluctuations in a lipid membranemay lead to transient anomalous diffusion of lipid molecules [1]. It is thereforeextremely interesting to test, whether anomalous diffusion due to critical fluc-tuations can be observed experimentally in model membranes under appropri-ate conditions. We report results of our experiments on model lipid membranesexhibiting near-critical fluctuations using STED-FCS [2], an experimentaltechnique which can provide valuable information on diffusion dynamics onspatial scales from a few tens to few hundreds of nanometers on time scalesranging from microseconds to seconds.[1] J. Ehrig, E. P. Petrov, and P. Schwille, Biophys. J. 100 (2011) 80.[2] L. Kastrup, H. Blom, C. Eggeling, and S.W. Hell, Phys. Rev. Lett. 94 (2005)178104.

484-Pos Board B270Interactions of Lithium Ions with Lipid MembranesTorri C. Roark1, Luis A. Palacio1, Philip A. Gurnev2, Bruce D. Ray1,Horia I. Petrache1.1Indiana University Purdue University Indianapolis, Indianapolis, IN, USA,2National Institutes of Health, Bethesda, MD, USA.Lithium is a naturally occurring alkali metal which is very easily ionized toLiþ, especially in aqueous solutions. Lithium is found mainly in mineral andseawater and in trace amounts in the human body. Lithium ions are beingused in various fields ranging from energy storage to the treatment of mentalillnesses including bipolar disorder and schizophrenia. The reason why lithiumis such a peculiar ion is not exactly known except that its mode of interactionswith charged surfaces depends on its hydration properties (i.e. interaction withnearby water molecules). We report two kinds of experimental measurementsof lithium interactions with charged lipid membranes. One set of experimentsinvolves X-ray scattering of multilamellar lipid vesicles in solution for whichwe measure how lithium salts modify van der Waals and electrostatic forcesbetween neighboring membranes. The other set of measurements compare

the effect of added lithium on the activity of gramicidin channels in neutraland negatively charged lipid bilayers. We find that in both x-ray and channelmeasurements, lithium ions modify lipid interactions in a manner that differsfrom other monovalent posititive ions.

485-Pos Board B271Superior Membrane Permeabilization and Solubilization Properties ofBiosurfactants may be Explained by Heterogeneous PerturbationMozhgan Nazari, Mustafa Kurdi, Badria Anis, Heiko Heerklotz.University of Toronto, Toronto, ON, Canada.Biosurfactants such as antimicrobial lipopeptides, saponins, and bile salts oftenshow a superior performance to permeabilize and lyse membranes and/or a bet-ter suitability for membrane protein solubilization than classical, synthetic de-tergents. We propose a classification of surfactants into those that arehomogeneously disordering versus heterogeneously perturbing a given mem-brane to help explaining and predicting this behaviour. Typical synthetic deter-gents such as C12EO8, octyl glucoside, SDS, and lauryl maltoside wereidentified as homogeneously disordering by the limiting fluorescence anisot-ropy of several DPH derivatives reaching a characteristic, low level at the onsetof solubilization. The biosurfactants surfactin, fengycin, iturin, digitonin, andlysophosphatidylcholine along with the synthetic CHAPS belong to anotherclass that initiates membrane lysis without critical disordering the whole mem-brane. They disrupt the membrane locally due to a spontaneous segregationfrom the lipid and/or an ordering effect that induces packing defects. Thismay account for enhanced activity, selectivity, and mutual synergism of anti-microbial biosurfactants. They should also be prone to form partially-demixed, asymmetric micelles (or bicelles) with a relatively lipid- (and sterol?)rich core surrounding a solubilized protein. Triton shows the pattern of a segre-gating surfactant in the presence of cholesterol. Zeta potential measurements ofliposomes exposed to surfactin show strong peptide-peptide interactions al-ready at a few mol-% in the membrane.

486-Pos Board B272Protection Against Photodamage of Biological Membranes: Effectivenessof Gallic Acid on Model Lipid BilayersOmar Mertins, Maurıcio S. Baptista, Rosangela Itri.University of Sao Paulo, Sao Paulo, Brazil.Biological membranes contain a large amount of unsaturated lipids which maybe easily degraded by action of singlet oxygen and free radicals generated bydegradation of molecules exposed to light irradiation. Lipid peroxidation leadsto changes in lipid bilayer permeability, fluidity and packing order [1]. In bi-ological media, such effects accelerate cell aging and culminate in cell death.Methylene blue (MB) is a powerful oxidizing agent due to a high quantumyield of singlet oxygen production when exposed to light of appropriate wave-length [2]. On the other hand, gallic acid (GA) interacts physically and chem-ically with singlet oxygen [3]. In this work we evaluate the ability of GA inreducing the damage of mimetic biological membranes caused by the actionof MB exposed to light irradiation. By means of phase contrast microscopy,it was possible to determine the best GA concentration to reduce the physicaldamage of the phospholipidic membrane of giant unilamellar vesicles and de-lay the permeability increase of the same. Furthermore, submitting the vesiclesunder adequate electric field [4], the increase of area per lipid as a result of ox-idation promoted by irradiated MB was quantitatively determined in the ab-sence and presence of different concentrations of GA. Results showa promising effect of GA in protecting against photo damage of biologicalmembranes.[1] E. Schnitzer, I. Pinchuk, D. Lichtenberg, Eur. Biophys. J. 36, 499 (2007).[2] W. Caetano, P. S. Haddad, R. Itri, et al., Langmuir 23, 1307 (2007).[3] A. Pajares, M. Bregliani, M. Montana, et al., J. Photochem. Photobiol. A209, 89, (2010).[4] K. A. Riske, T. P. Sudbrack, N. L. Archilha, et al., Biophys. J. 97, 1362(2009).

487-Pos Board B273Combined Brewster Angle and Fluorescence Microscopy of DMPC/D-Cholesterol Mixed Langmuir MonolayersPritam Mandal, Fanindra Bhatta, Edgar E. Kooijman, David W. Allender,Elizabeth K. Mann.Kent State University, Kent, OH, USA.Most direct imaging studies of domains and of the mixing/demixing transitionwithin lipid monolayers and bilayers are performed with fluorescence micros-copy (FM). This technique requires the addition of a fluorescent probe, whichcan in principle affect the size, shape and behavior of the domains, as well as